network

Well you have one part of the solution already stated, you need a router between thw two networks. I will leave it up to you a Google to learn a bit mor about that part.

As far as calling from one to the other goes, depending on what protocol you are using (ie: SIP or H.323) you are goign to need somethign that allows you to resolve soem kind of dial plan so you know what is on one side of each network and then how to cross over to the gateway when needed. Look up thing slike either a SIP proxy or a H.323 gatekeeper.